In order to increase data transfer rate and system capacity, next-generation mobile communication and wireless transmission systems use a Multi-Input Multi-Output (MIMO) scheme in which data is transmitted using multiple antennas.
Single-user MIMO forms a channel matrix H between one Mobile Station (MS) having two or more antennas and a Base Station (BS) having two or more antennas. On the other hand, multi-user or collaborative MIMO forms a channel matrix H between two or more MSs, each having one antenna, and one BS having multiple antennas. The concept of uplink collaborative MIMO is illustrated in FIG. 1. This method has several advantages. First, each MS needs only one power amplifier since only one transmission path is necessary for the MS. In addition, even though each MS operates with other MSs in a MIMO scheme, it is not necessary to divide an output of the MS into respective signals for antennas since the MS transmits signals using one antenna, and thus the MS need not undergo a 3 dB power loss that is common in general MIMO. Second, if two MSs are appropriately selected as described above, it is possible to obtain a much better channel matrix H than when two antennas are installed in one MS.
In the first-generation MIMO scheme, it is assumed that one BS and one user (MS) transmit a plurality of information simultaneously through multiple antennas. On the other hand, in the multi-user MIMO scheme, one BS and multiple users (MSs) create a matrix for MIMO Spatial Multiplexing (SM) as can be seen from FIGS. 1 and 2. It can be understood from the basic concept of MIMO that the channel characteristics matrix for MIMO SM is further improved as the correlation between inner values of the matrix decreases. Accordingly, when multiple users are considered at once, it is possible to obtain a matrix with better channel characteristics since the antennas of the users are expected to have a lower correlation therebetween.
Particularly, if only the single-user MIMO scheme should be used in the example of FIG. 2, the BS should determine an MS with which the BS should form a 2×2 matrix at a specific time in order to achieve the highest transfer rate. It is possible to achieve a transfer rate of a total of 2.5 Mbps (2.0+0.5) if the BS forms a MIMO channel with User-1, to achieve a transfer rate of a total of 2.0 Mbps if the BS forms a MIMO channel with User-2, and to achieve a transfer rate of a total of 1.8 Mbps if the BS forms a MIMO channel with User-3. Accordingly, the BS forms a single-user MIMO channel with User-1 at the given time.
However, in the case where the multi-user MIMO scheme is used, it is possible to determine that signals carried over different transmit antennas are signals for transmission to different users. Therefore, in this case, it is possible to achieve a transfer rate of a total of 3.5 Mbps (2.0+1.5) by carrying a signal for transmission to the User-1 over the antenna #1 and carrying a signal for transmission to the User-2 over the antenna #2.
Even in the multi-user MIMO system, a MIMO channel may be formed between the BS and a single user in the case where a transfer rate that can be achieved between the BS and the user at a specific time is the highest among all combinations of transfer rates. Thus, it can be understood that the multi-user MIMO is conceptually broader than the single-user MIMO.
The conventional technologies use a single-user power control scheme in which it is assumed that only one user uses a specific resource, regardless of the number of users that transmit signals by simultaneously using the same resource. However, actually, a number of users may use the same resource as in the multi-user MIMO scheme.
However, in a wireless mobile communication system, as the number of users increases, the sum of powers of signals transmitted from users increases and thus the interference between communication devices in the system increases. The increased interference may reduce reception performance of the BS. In addition, in multi-cell environments, if the number of MSs which use the same resource in a specific cell increases, interference with MSs which use the same resource in a neighbor cell adjacent to the specific cell may increase. Accordingly, when a number of users transmit signals through the same resource, a factor in consideration of such transmission through the same resource should be reflected in the power control scheme.